A compound crystal of Group 13 metal and nitride, as represented by gallium nitride (GaN), is useful as a substance for use in light-emitting diodes, laser diodes, high frequency-capable electronic devices and the like. In the case of GaN, the size of the GaN crystal produced by a known method is about 10 mm at present (see, Oyo Butsuri (Applied Physics), Vol. 71, No. 5, page 548 (2002)) and this is insufficient for the application to a semiconductor device. As for the practical production process of a GaN crystal, a method of effecting a vapor phase epitaxial growth on a sapphire substrate or a substrate such as silicon carbide by an MOCVD (metal-organic chemical vapor deposition) process has been proposed (see, for example, J. Appl. Phys., Vol. 83, pp. 764-767 (1998)).
However, in the above-described method, the GaN crystal is epitaxially grown on a heterogeneous substrate differing in the lattice constant and thermal expansion coefficient and therefore, many lattice defects are present in the obtained GaN crystal. When such a GaN crystal allowing for the presence of many lattice defects is used, an adverse effect is caused on the activity of the electronic device, and satisfactory performance cannot be expressed for use in the applied field such as blue laser. Therefore, improvement of the quality of GaN crystal grown on a substrate and establishment of the technique for the production of GaN bulk single crystal are being strongly demanded.
At present, in the heteroepitaxial GaN crystal growth method by the vapor phase process, a complicated long step is required so as to decrease the defect concentration of GaN crystal. Therefore, aggressive studies are recently being made on the formation of GaN single crystal, and there have been proposed a high-pressure method of reacting nitrogen and Ga at a high temperature under a pressure (see, J. Crystal Growth, 178, page 174 (1997)), a method of reacting Ga and NaN3 while elevating the pressure (see, H. Yamane; Preparation of GaN single crystals using a Na flux, Chem. Mater., pages 413-416 (1997), a flux growth method (see, Oyo Butsuri (Applied Physics), Vol, 71 , No. 5, page 548 (2002); J. Crystal Growth, 260, page 327(2004); and Kinzok (Metals), Vol. 73, No.11, page 1,060 (2003), and the like, As for the flux, an alkali metal is often used, but the crystal growth rate is low and only a plate-like crystal having a size of about 10 mm can be obtained. Moreover, many unclear points are remaining, such as crystal growth mechanism or reason why the crystal growth stops at the size of about 10 mm. On the other hand, it has been attempted to produce GaN by oxidizing nitrogen ion on the Ga surface serving as an electrode in a molten salt (see, 29-Kai You-Yu En Kagaku Toren-Kai Yoshi Shu (Summary Collection at 29th Chemical Discussion on Molten Salt), page 11 (1997)), but an industrially realizable process is not yet established. Furthermore, a method of synthesizing GaN by an ammonothermal process has been reported (see, Acta Physica Polonica A, Vol. 88, page 833 (1995)), but the obtained GaN crystal has a problem, for example, in the crystal size or number of lattice defects and this method is not practiced in industry.
As described above, in the heteroepitaxial crystal growth method on a substrate by a vapor phase process, a Group 13 metal nitride crystal reduced in the lattice defect cannot be obtained. In other methods using a high pressure, the apparatus becomes large-scaled and the profitability is low. In the ammonothermal method using an ammonia in a supercritical state, the apparatus and materials used are very expensive.